One-Way Audio on VoIP Calls: Causes and Fixes

What Is One-Way Audio?

One-way audio is one of the most common VoIP problems. It occurs when one party on a call can hear the other, but the audio only flows in one direction. Typically, you can hear the person you called, but they cannot hear you — or vice versa.

One-way audio is almost always caused by a network configuration issue rather than a fault with your phone or VoIP service. The good news is that it is usually straightforward to fix once you identify the cause.

Common Causes of One-Way Audio

1. SIP ALG Is Enabled on Your Router

SIP ALG (Application Layer Gateway) is the number one cause of one-way audio on VoIP calls. SIP ALG modifies the IP addresses in SIP packets as they pass through your router, which causes the RTP audio stream to be sent to the wrong address. The result is that audio flows in one direction but not the other.

SIP ALG is enabled by default on most routers. See our detailed guide on how to disable SIP ALG.

2. NAT/Firewall Blocking RTP Ports

VoIP calls use two separate protocols — SIP for call signalling and RTP for the actual audio. If your firewall allows SIP traffic (so calls connect) but blocks RTP traffic (UDP ports 10000–20000), the call will connect but audio will not flow in one or both directions.

Check that your firewall allows both inbound and outbound UDP traffic on ports 10000–20000. See our firewall configuration guide for detailed port requirements.

3. Incorrect NAT Configuration

When your phone is behind a NAT router, it may advertise its private IP address (e.g. 192.168.1.100) in SIP packets instead of your public IP address. The remote party then tries to send audio to this private address, which is unreachable from the internet.

To fix this, configure your PBX or phone with your external/public IP address or enable STUN (Session Traversal Utilities for NAT) so the device can discover its public address automatically.

4. Codec Mismatch

If the two endpoints on a call cannot agree on a common audio codec, the call may connect (SIP signalling succeeds) but audio will not flow. Ensure your phone and provider support at least one common codec — G.711a is the safest universal choice. See our codec guide for more details.

5. Network or Routing Issues

Less commonly, one-way audio can be caused by asymmetric routing on your network, where outbound and inbound traffic take different paths. This can occur on networks with multiple internet connections or complex routing configurations.

Step-by-Step Fixes for One-Way Audio

Work through these fixes in order — the most common causes are listed first:

Step 1: Disable SIP ALG

Log into your router's admin interface
Find the SIP ALG or SIP Passthrough setting
Disable it completely
Save settings and reboot the router
Reboot all VoIP phones after the router restarts

This single step resolves one-way audio in the majority of cases.

Step 2: Open RTP Ports on Your Firewall

Ensure UDP ports 10000–20000 are open for both inbound and outbound traffic
If you use a restrictive firewall, whitelist your VoIP provider's media server IP ranges
Verify the rules are active and applied to the correct network interface

Step 3: Check NAT Settings

Configure your PBX or phone with your public IP address in the NAT/external IP settings
Enable STUN on your phone or PBX if available
Ensure SIP keep-alive packets are enabled to maintain NAT mappings

Step 4: Verify Codec Configuration

Check that your phone has G.711a (PCMA) enabled as a supported codec
Ensure your provider supports the codecs configured on your phone
If in doubt, set G.711a as the only codec temporarily to rule out codec issues

Step 5: Test on a Different Network

Connect your phone to a mobile hotspot to bypass your office network entirely
If audio works correctly on the hotspot, the issue is confirmed as a network/firewall problem
If the issue persists on a different network, contact your VoIP provider

When to Contact Your Provider

If you have worked through all the steps above and still experience one-way audio, the issue may be on your provider's side. Contact them with the following information:

The time and date of affected calls
The numbers dialled and the direction of the missing audio
Your public IP address
Confirmation that SIP ALG is disabled and firewall ports are open

Your provider can analyse SIP traces and RTP streams to pinpoint the exact cause. Contact our support team if you need assistance.